Perceptual grouping is the process by which the initially raw and inchoate visual image is organized into perceptual "objects". What spatial factors induce perceptual grouping? What is the sequence of computations whereby the image is progressively organized? One source of difficulty in modeling this process is that, unlike many aspects of early vision, perceptual grouping inherently involves non-local: computations - integration of cues from potentially distant locations in the image. Another difficulty in understanding perceptual grouping has been the lack of objective and temporally precise methods for actually measuring the observer's subjective organization of an image. This proposal seeks to combine (a) recent advances in understanding the non-local computations involved in perceptual grouping with (b) novel experimental methods for determining subjective organization. The experimental methods are based on the finding that perceptual objects enjoy certain objectively measurable benefits, including more efficient visual comparisons within them than between distinct objects. This proposal seeks to use this effect to discover what the visual system in fact treats as a perceptual object, and how this percept develops over the course of processing. Most of the proposed experiments involve carefully constructed artificial stimuli with various grouping cues in force, designed to allow detailed comparisons of the strength, interaction, and time-course of each potential grouping cue. In addition, several experiments involve natural images, in order to uncover how perceptual organization proceeds under more naturalistic conditions. This research may lead to technological advancement in the area of computer vision, as well as to better understanding of disorders of perceptual organization such as visual agnosia and dyslexia. [unreadable] [unreadable]